This invention relates to the joining of materials together, and, more particularly, to joining accomplished by soldering.
Soldering is a metal joining process wherein portions of pieces are joined together by placing a thin layer of a molten metal between the portions being bonded, and permitting the metal to solidify. The usual steps in soldering involve physical cleaning of the surfaces to be joined, applying a solder flux to the surfaces, heating the surfaces to cause the flux to react with oxides and other surface layers, thereby cleaning away these surface impediments to bonding, applying a layer of the molten solder to each surface to wet the surface in a process termed "tinning", contacting the two surfaces together with the solder molten, and finally postcleaning the surfaces to remove any corrosive remnants of the flux. It is important that the molten solder fully wet the surfaces to be joined during the tinning step, as unwet portions result in voids at the surfaces after soldering is complete, and the voids can cause later failure of the bond.
In soldering, usually only the portions to be bonded are heated, and the solder melts at low temperature. In a related process, brazing, usually the entire part is heated, and the braze metal melts at higher temperatures. As used herein, the term "soldering" includes and encompasses both the process usually termed "soldering" as well as the process usually termed "brazing".
The flux and its application are an important part of the soldering process. Solders cannot bond to oxides and contaminants that form barriers at the surfaces of the pieces to be joined. Such surface barriers prevent wetting of the surfaces during tinning, resulting in incomplete bonding. The flux is a chemical substance that contains an ingredient which reacts with oxides and other surface barriers to remove them and inhibit their reformation during the soldering process. With these barriers removed, the liquid solder can flow over and wet the entire surface of the portion to be joined during tinning. Once such wetting is accomplished, the actual joining of the two components is readily completed.
The surface to be fluxed must ordinarily be cleaned of a portion of the oxide and contaminant coating prior to fluxing. Equally importantly, the soldered array must be carefully cleaned after soldering is complete, in a postcleaning process, to remove any unreacted flux and reaction products of the flux treatment. Any remaining flux or reaction products can continue reacting with the bonded pieces and corrode them during service. Such corrosion caused by fluxes is a major concern for many types of solder bonding, as the bonded pieces are sometimes used in environments having conditions such as high humidity and high temperature that accelerate corrosion reactions that might otherwise proceed slowly. Tiny amounts of unreacted flux or reaction products that remain on the surface following soldering and postcleaning can therefore have disastrous consequences.
The problems of attaining good soldered joints and the consequences of failures are keenly felt in the electronics industry. Even with the advent of integrated circuit technology, soldering is widely used in assembling electronic devices. A large, complex electronic device may contain thousands of solder joints. These solder joints must function both to structurally join pieces and to carry electrical currents. Failure of any one of the joints, either because of poor wetting and bonding of the solder or because of corrosion, can result in failure of the device. The electronic devices are often placed or used in inaccessible locations, so that a failed solder joint cannot be readily repaired.
The soldering operation itself is challenging when applied to electronics. The pieces being joined are often small, making cleaning, tinning, postcleaning and inspection difficult. In some cases, portions of the electronic components are damaged by overheating, and only the lead portions to be joined can be heated during soldering. Cleaning and postcleaning are difficult due to the small sizes of the components, their large numbers, and the potential damage to the electronics by the cleaning solutions used, if any. Entire production line shutdowns due to problems in the joining operation are not uncommon, and such problems are often traceable to the fluxes used and the cleaning, postcleaning, and application steps associated with fluxing.
Because of the consequences of solder failures and the significance of solder joints, the soldering process has been studied extensively. Various fluxes have been developed, but the natural rosin familiar to home hobbyists is still most commonly used in production soldering operations. Natural rosin is extracted from pine tree sap, and contains a mixture of chemicals that effectively react to remove many oxides and contaminants at surfaces. Studies of the content of natural rosins have led to the development of various synthetic fluxing agents, which usually perform functions similar to those of natural rosins, but without the side effects. However, most effective fluxing agents require extensive cleaning and post cleaning, as they leave residue or reaction products that can lead to corrosion of the portions bonded.
Fluxes now in use include mixtures of inorganic acids in inorganic vehicles or solvents such as water, inorganic acids in petrolatum pastes, salts in water, petrolatum paste or organic solvent, organic acids in water, organic solvents or petrolatum paste, organic halogens in water, organic solvents, or petrolatum paste, amines and amides in water, organic solvents, or petrolatum paste, and natural or modified rosins. All of these fluxes require postcleaning of the soldered joint. Fluxing can also be carried out in a reducing environment, as by accomplishing the soldering process in a hydrogen gas atmosphere. Postcleaning is not required, but maintenance and use of a pure hydrogen atmosphere in large-scale production operations is difficult, and can lead to related hydrogen embrittlement. In sum, there is known no effective, generally applicable approach to fluxing which avoids the need for postcleaning of the soldered parts.
There exists a need for an improved approach to soldering, and particularly to the fluxing operation, which increases the reliability of the joining process by promoting effective wetting of the solder to the piece during the tinning step, and also reduces the incidence of post-soldering failures due to corrosion or related causes. Preferably, such an approach would be accomplished through a relatively simple sequence of soldering steps, and in a manner that is both environmentally acceptable and also does not require special precautions to protect the health and productivity of production line workers. The present invention fulfills this need, and further provides related advantages.